1. Field of the Invention
The invention relates to a method of correcting vignetting in the upper or lower range of the pick-up area of optoelectric transducers of a color television camera comprising a zoom lens, which vignetting is produced in dependence upon the zoom setting and when an objective-specific minimum diaphragm aperture value is exceeded, in which method a representative signal is derived from the diaphragm aperture setting and from the zoom setting.
2. Description of the Related Art
It is known that the optical input of a color television camera, i.e. both the camera lens, particularly the zoom lens at different diaphragm apertures, and the beam splitter produce different vignettings on the light-sensitive surface of each optoelectric transducer of a multichannel color television camera so that the image displayed on the display screen has a varying brightness, for example, it is brighter in the center than at the edges. These vignettings, which have approximately equal effects on each pick-up area, can be generally compensated for electronically by controlling the video signals with line and field-frequency sawtooth and parabolic voltages.
When a zoom lens is used, a further vignetting can be observed when the diaphragm aperture increases by a given value, for example from 2.8 to 1.4, dependent on the zoom setting, and the optoelectric transducers in the different chrominance channels are differently illuminated, both in wide-angle setting and in telesetting. In wide-angle setting the optoelectric transducers are illuminated in the upper area to a lesser extent than in the lower area due to the optical limitation of the exit pupil of the objective and the effect of the dichroic layers of the beam splitter at different angles of incidence of the light beam in the blue and red chrominance channels, whereas in the green chrominance channel the optoelectric transducer is illuminated in the upper area to a greater extent than in the lower area. In contrast, as from a given telesetting value, the optoelectric transducers are illuminated in the upper area to a greater extent than in the lower area due to the optical limitation of the entrance pupil of the objective and also in connection with the effect of the beam splitter in the blue and red chrominance channels, whereas the optoelectric transducer is illuminated conversely in the green channel. Consequently, when picking up a white scene as from an objective-specific diaphragm aperture and wide-angle setting, the upper area will be displayed greenish on the display screen of a color television monitor and the lower area will be displayed to a certain extent in magenta. In telesetting, these discolorations of the lower and upper areas of the display screen are reversed so that the upper area appears in magenta and the lower area appears greenish. The discolorations will be stronger as the diaphragm aperture is larger and as the zoom setting is more extreme.